1. Field of the Invention
The invention relates to communication systems, and more particularly to an apparatus for achieving frame synchronization in a digital receiver.
2. Related Technology
In radio transmission, information is conveyed by uniformly spaced pulses and the function of any receiver is to isolate these pulses as accurately as possible. However, due to the transmission channel, the received signal has undergone alterations during transmission, and a complete estimation of certain reference parameters is necessary prior to data detection. These unknown parameters can cover such factors as the optimum sampling location, the start of a data packet (for burst mode transmission) or of a frame marker for continuous transmission, or the phase offset introduced in the channel or induced by instabilities between the transmitter and receiver oscillators. The extraction of the phase or frequency of the incoming carrier is known as phase/frequency estimation. Alternatively, non-coherent demodulation such as differential demodulation can be applied where the phase difference between one data symbol and the next is assumed constant.
In traditional analog receivers, synchronization of the phase and frequency is typically performed in the intermediate frequency (IF) stage of the receiver. However, the IF analog components are costly and prone to undesirable variations over time. Flexibility in the design of the receiver synchronization unit has increased in recent times with the advent of increasingly powerful silicon chips, which are considerably cheaper and more stable. This has led to a reduction in the amount of signal processing being performed at IF. In the current state of the technology, IF sections are reduced to an asynchronous sampling device for analog to digital conversion and a free-running oscillator for down conversion to baseband. The term “baseband” refers to when the carrier frequency has been completely removed from the received signal and the signal is centered at DC (0 Hz). In typical digital receivers, the asynchronous sampling device operates at a rate of two or more samples per symbol. The term “symbol” is used in this context to refer to transmitted signals that are phase modulated with discrete phase and or amplitude relationships. Each assigned phase and or amplitude relationship is a symbol that is subject to detection at the receiver.
In communication systems, information is transmitted either continuously or in bursts. In both cases, the data from the information source at the transmitter is sub-divided into units known as frames. The purpose of data frames in continuous transmission is to provide a marker to track the received data at the end-user destination as well as to organize the data stream into uniformly sized groups of bits. Even more importantly, frame synchronization is essential in any system utilizing block error control coding (H. Meyr, M. Moeneclaey and S. Fechtel, “Digital Communication Receivers: Synchronization, Channel Estimation and Signal Processing”, John Wiley Publishers, 1998, pp. 542-545) wherein codewords are identified with respect to the frame synchronization reference. Moreover, frame synchronization is very important in continuous transmission when frames can be lost due to adverse channel conditions if the receiver cannot track and remove the condition quickly enough. When the receiver settles again, there should be some mechanism to indicate to the receiver when the detected data is meaningful. This function is performed by the periodic insertion of frame markers and midambles in packet transmission to indicate the start of valid data and to assist in updating the parameter acquisition and tracking mechanism. In burst mode transmission, the data bursts are received starting at a random location within a predefined time slot. The purpose of frame synchronization in this case, as well as before for continuous data, is to estimate the location of the start of the data as well as assisting in the estimation of the unknown parameters for the receiver detection.
The most common technique used in frame synchronization is the insertion of fixed data patterns at the transmitter, known as frame markers, at the start of the data frame to assist synchronization. The purpose of frame synchronization is to isolate the position of the start of the arbitrary data stream, which follows these frame markers, as illustrated in FIG. 1. A compromise is necessary between the length of the frame marker to ensure minimal loss of synchronization and the length of the associated information bits in the frame to achieve an efficient data throughput. Data throughput refers to the amount of information bits sent in a frame with respect to the total number of bits sent in the frame.
To achieve frame synchronization at the receiver using the frame marker method, the receiver searches the entire data stream for a sequence matching the known frame marker inserted at the transmitter. From a signal theory perspective, the receiver performs a cross correlation of the frame marker with the received signal. If the receiver is not in synchronization with the framing pattern, the accumulated correlation will be low. When the receiver comes into frame synchronization, however, the correlation should be nearly perfect, blemished only by an occasional detection error. Synchronization is achieved by implementing a filter with the values of the coefficients at the filter taps matched to the frame marker sequence inserted at the transmitter. Depending on the sampling rate N, the filter taps are spaced N delays apart to isolate the correct sample at which the frame marker sequence ends (the value of N is the same). Matching the coefficients at the transmitter and receiver ensures that the correlation energy is maximized at the filter output when the two sequences coincide. The frame marker sequence in the literature is also referred to as a unique word or synchronization sequence; hereafter the frame marker sequence is referred to as the unique word due to its special correlation properties.
The unique word sequence is chosen for its near-optimum correlation properties, a Dirac (or impulse) auto correlation characteristic is ideal for frame synchronization. However, in practice, the auto-correlation characteristic of a unique word sequence contains a strong peak where the two sequences coincide as well as sidelobes at fixed intervals on both sides of the main peak as illustrated in FIG. 2. FIG. 2 shows the situation where the input signal is sampled at one sample per symbol, which implies there is only one sample on the main lobe of the correlation. A good unique word has the property that the absolute value of its correlation sidelobes is small with respect to the absolute value of the main correlation lobe. A correlation sidelobe is the value of the correlation of the unique word with a time-shifted version of itself.
The next stage in any receiver is the detection of the correlation peak. In the case of complex modulation schemes where data is transmitted on both an In-phase (I) branch and a Quadrature (Q) branch, the unique word is simultaneously transmitted on both branches. For complex modulation schemes where the unique word is repeated on both the I and Q branches, a complex matched filter is unnecessary. Instead two real matched filters outputs for both the in-phase and quadrature components are combined to yield the equivalent complex matched filter output. This avoids unnecessary complexity in the receiver implementation. Therefore, to obtain the overall autocorrelation function, the magnitude or magnitude squared of the I and Q correlation outputs is taken. The magnitude of the correlation eliminates the effect of any phase offset present on the received signal at the input to the correlators. This technique gives reliable results for the case where the phase offset is of the order of 10−3 of the inverse of the midamble (or unique word if no midamble is used) duration. The next step is to pass the absolute value of the correlation through a threshold detector. The threshold detector is set to a sufficiently high value such that only the main lobe of the correlation passes the threshold.
Typically in the technology, frame synchronization occurs after the receiver has compensated for the phase, frequency and timing offsets on the received signal. In this situation, the input signal contains no phase jitter and therefore, a short unique word is sufficient to obtain reliable frame synchronization. However, in the current technology there is a trend to avoid the transmission of known data streams with limited functionality (H. Meyr, M. Moeneclacy and S. A. Fechtel, “Digital Communication Receivers: Synchronization, Channel Estimation and Signal Processing”, John Wiley Publishers, 1997, pp. 486-488) so the unique word sequences assist in other aspects of the receiver, such as phase and timing estimation. This implies that frame synchronization in these systems occurs before timing and phase estimation (O'Shea et. al., “Joint Maximum Likelihood Frame and Timing Estimation for a Digital Receiver”, U.S. patent application Ser. No. 09/093,414, filed Jun. 8, 1998).
However, performing frame synchronization prior to phase and timing estimation involves careful design of the frame synchronization unit. In these conditions, the input signal to the frame correlation unit is sampled at a rate of at least two samples per symbol since timing estimation, in general, requires at least two samples per symbol (H. Meyr, M. Moeneclaey and S. A. Fechtel, “Digital Communication Receivers: Synchronization, Channel Estimation and Signal Processing”, John Wiley Publishers, 1997, pp. 283-295). As an example, if the rate is two samples per symbol, the main lobe is sampled at two samples per symbol as illustrated in FIG. 3. Comparing the main lobe of FIG. 3 to that of the main lobe in FIG. 2, only one sample of the main lobe is available in FIG. 2, whereas two samples are available in FIG. 3. Due to asynchronous sampling, the sampled absolute value (or magnitude squared value) of the correlation output differs depending on the instant where sampling occurs as illustrated in FIG. 4, which also shows that for two samples per symbol, two values can cross a conservative setting for a threshold.
When frame synchronization occurs prior to phase estimation, the presence of a phase offset has a significant impact on the setting of the threshold as the correlation peak value varies depending on channel conditions. This results in conservative values being set for the threshold detector level, which can lead to two values crossing the threshold as illustrated in FIG. 4. The issue is then to isolate the true location of the start of the arbitrary data. What is needed is a circuit that improves the flexibility of the threshold circuit by isolating the correct location of the beginning of the arbitrary data when more than one sample crosses the threshold detector. Such a circuit would improve the threshold design by using information about the threshold input (i.e., whether the signal has a rising slope) to improve reliability in the presence of phase and frequency offsets.
In the current technology, frame synchronization is discussed either in terms of CDMA (Code Division Multiple Access) or TDMA (Time Division Multiple Access) schemes. Frame synchronization and detection techniques for alternative transmission multiple access schemes, which transmit all users simultaneously, have not been discussed. As an example, schemes that combine aspects of both CDMA and TDMA, such as a joint TDMA/CDMA scheme, have not been addressed. The term “multiple access schemes” defines the various ways in which multiple users can access a common channel to transmit data. In one joint TDMA/CDMA scheme (Masood K. Tayebi, “Wireless Multimedia Carrier System”, U.S. patent application Ser. No. 08/954,217, filed Oct. 20, 1997), rather than assigning a unique spreading code to each user, all users are assigned a common spreading code equal to the length of the number of users. However, each user is assigned a unique time offset equal to the duration of one chip with respect to the start of the previous user's data stream as illustrated in FIG. 5. The term “chip” refers to one bit of the pseudo-noise spreading code so as to differentiate it from the source data bits. The data streams are then summed together to form the transmitted signal. At the receiver, following a pulse shaping filter, the spreading code is removed using a matched filter, such as a spreading code matched filter, or any of a variety of techniques available in the literature (J. G. Proakis, “Digital Communications”, McGraw-Hill Publishers, Third Edition, 1995, pp. 744-752). However, any alternative technique can be used with equal success. Due to the shifts at the transmitter of the users by one spreading chip interval, the users'original data bits, following the spreading code matched filter, are now transformed to a data sequence at the spreading code rate where each user in turn contributes one data chip. The data sequence corresponds to the ordering of the users, i.e., the first chip is from user one, the second from user two and so on. As the number of users equals the length of the spreading code, the first three chips of the data sequence are of duration equal to one data bit. A sidelobe suppression filter is used to suppress undesirable sidelobes following the spreading code matched filter.
What is desired is a method for frame synchronization suitable for a joint TDMA/CDMA scheme or similar multiple access schemes that provide simultaneous frame synchronization of multiple users with minimum overhead bandwidth. Instead of a separate unique word for each user, which is wasteful of bandwidth, it would be desired to distribute the unique word bits over all the users. In this situation the unique word would be chosen as some integer multiple of the number of users. For example, in a TDMA/CDMA system with 32 users, at the start of each user's stream there would be two bits pre-assigned for the unique word. This would avoid the transmission of a 64 bit unique word on each user's data stream. The output of the spreading code matched filter at the start of the data frame would be a 64 chip unique word. Each bit of the unique word would appear contiguously in slots of duration equal to the spreading code chip length. Such a proposed scheme would also be adopted for more traditional multiple access schemes. What is also desired is a robust method of threshold detection for situations when the input signal to the frame synchronization unit would be over-sampled at a rate higher than one sample per symbol and may contain a frequency and/or phase offset.